Process of producing nickel chromium alloy products

ABSTRACT

WROUGHT CORROSION-RESISTANT NICKEL-CHROMIUM ALLOY PRODUCTS HAVING HIGH STRENGTH AND DUCTILITY AT ELEVATED TEMPERATURES ARE PRODUCED BY HOT-WORKING A TWO-PHASE NICKELCHROMIUM ALLOY CONTAINING, BY WEIGHT, FROM 47 TO 65% CHROMIUM, FROM 0 TO 12% COBALT, FROM 0.02 TO 0.1% CARBON, FROM 0 TO 0.01% BORON, ONE OR MORE OF TITANIUM, ALUMINUM, MOLYBDENUM, TUNGSTEN, TANTALUM AND NIBOIUM IN AMOUNTS IN THE RANGES FROM 1 TO 6% TITANIUM, FROM 0.5 TO 5% ALUMINUM, FROM 1 TO 10% MOLYBDENUM, FROM 2. TO 10% TUNGSTEN, FROM 2 TO 10% TANTALUM AND FROM 0.5 TO 10% NIOBIUM, AND SUCH THAT   3 X (PERCENT TI) + 3 X (PERCENT AL) + 5 X (PERCENT MO) + 2.5 X (PERCENT W) + 2.5 X (PERCENT TA) + 5 X (PERCENT NB) - 12 $ 0   WHICH THE PROVISOS THAT (I) THE SUM OF THE ALUMINUM AND TITANIUM CONTENTS DOES NOTE EXCEED 6%; (ii) THE SUM OF THE ALUMINUM, TITANIUM, NIOBIUM AND TANTALUM CONTENTS DOES NOT EXCEED 15%; AND (iii) THE SUM OF THE TUNGSTEN AND MOLYBDENUM CONTENTS DOES NOT EXCEED 10% AND ONE OR MORE OF ZIRCONIUM, CERIUM YTTRIUM AND HAFNIUM IN A TOTAL AMOUNT EXCEEDING THAT REQUIRED TO COMBINE WITH ALL THE NITROGEN PRESENT AND SUFFICIENT TO FORM A EUTECTI WITH THE NICKEL, BUT NOT EXCEEDING 4%, THE BALANCE, EXCEP FOR IMPURITIES INCLUDING NITROGEN IN AN AMOUNT NOT EXCEEDING 0.1%, BEING NICKEL IN AN AMOUNT OF AT LEAST 25%, TO BREAK DOWN ITS CAST STRUCTURE AND THEN SOLUTION-HEATING IT AT A TEMPERATURE WITHIN THE RANGE OF 50*C. BELOW ITS SOLIDUS TEMPERATURE FOR FROM 1/2 TO 20 HOURS.

United States Patent Oifice 3,811,960 Patented May 21, 1974 PROCESS OFPRODUCING NICKEL CHROMIUM ALLOY PRODUCTS Philip James Parry, Birmingham,and Peter Lindsay Twigg, Worcs, England, assignors to The InternationalNickel Company, Inc., New York, NY.

No Drawing. Filed Jan. 11, 1973, Ser. No. 322,610 Claims priority,application Great Britain, Jan. 17, 1972, 2,170/ 72 Int. Cl. C22f 1/10,N11

US. Cl. 148-115 F 6 Claims ABSTRACT OF THE DISCLOSURE 3 X (percent Ti)+3(percent Al) +5 (percent Mo) 2.5 X (percent W) +2.5 (percent Ta) +5(percent Nb) 12 O with the provisos that (i) the sum of the aluminum andtitanium contents does not exceed 6%;

(ii) the sum of the aluminum, titanium, niobium and tantalum contentsdoes not exceed 15%; and

(iii) the sum of the tungsten and molybdenum contents does not exceed10% and one or more of zirconium, cerium, yttrium and hafnium in a totalamount exceeding that required to combine with all the nitrogen presentand sufiieient to form a eutectic with the nickel, but not exceeding 4%,the balance, except for impurities including nitrogen in an amount notexceeding 0.1%, being nickel in an amount of at least 25%, to break downits cast structure and then solution-heating it at a temperature withinthe range of 50 C. below its solidus temperature for from /2 to 20hours.

This invention relates to wrought nickel-chromium alloys for use atelevated temperatures in corrosive environments and which have animproved combination of stressrupture strength, ductility and corrosionresistance.

Nickel-chromium alloys containing from 45 to 75% chromium are known tohave good resistance at elevated temperatures, e.g. 600 C. and above, tocorrosion by the combustion products of low-grade fuel oil, whichproducts include highly corrosive compounds of sodium and sulphur, andin particular of vanadium. At these high chromium contents the alloyshave a two-phase structure on solidification, consisting of thealpha-chromium and gamma-nickel phases. In the cast form they exhibitmoderate strength at such temperatures, but suffer from lack ofductility, and they are not readily workable into wrought products.

Various attempts have been made to improve the properties ofnickel-chromium alloys.

The workability and ductility of alloys containing from 28 to 75chromium, with or without up to 49% iron, and including the two-phasealloys with 45% or more chromium, can be improved, as described andclaimed in the specification of our Pat. No. 3,627,511, by incorporatingin the alloy one of the eutectic-forming elements zirconium, cerium,yttrium and hafnium in excess of the amount required to combine withnitrogen in .the' alloy as nitride and adequate to form a eutectic withthe nickel. These additions enable the cast alloys to be wrought and theresulting wrought products to be further worked. The ductility of thewrought alloys at temperatures below 600 C. can be further increased byannealing in the temperature range of 600 to 800 C. Whether or not thisheattreatment is used, the stress-rupture strength of the alloys attemperatures above 600 C. remains fairly low, and it is desirable toproduce wrought alloys having an improved combination of stress-rupturestrength, ductility and corrosion resistance at temperatures in excessof 600 C.

In the specification of our Pat. No. 3,519,419 we have described aprocess in which nickel-chromium alloys which consist essentially of asingle gamma solid solution phase containing precipitable alpha-chromiumare subjected to a combined mechanical and thermal treatment to producea micro-duplex gamma-alpha microstructure, which can be strengthened bythe precipitation of inter-metallic gamma prime phases containingtitanium, aluminum, niobium or tantalum or by the presence ofsolid-solution strengthening elements such as molybdenum or tungsten.However the need to obtain a single phase structure limits the amount ofchromium that can be present or requires the presence of iron, both ofwhich impair the resistance of the alloys to corrosion byvanadium-containing combustion products of impure hydrocarbon fuels. IThe well-known nickel-chromium superalloys containing up to about 35%chromium together with one or more precipitation-hardening or solidsolution-strengthening elements also have a single-phase austenitic(gammaphase) matrix. The heat treatments commonly used to strengthensuch alloys comprise a solution heating step followed by an agingtreatment, the solution heating being performed at the lowest practicaltemperature in order to avoid loss of creep ductility at elevatedtemperatures.

Another proposal for improving the properties of cast.

nickel-chromium alloys was made by Bloom and Grant in US. SpecificationNo. 2,809,139. These patentees postulated the existence innickel-chromium alloys with more than 40 or 45% chromium of ahigh-temperature chr0- mium phase which they called the beta-phase,stable above an eutectoid temperature of 1180 C. Their proposed methodof improving the stress-rupture strength of the cast alloys at atemperature of 816 C. comprised heating the alloys above the eutectoidtemperature followed by cooling, with or without subsequent heatingbelow this temperature to develop a fine-grained high temperaturestructure. Subsequent work has failed to demonstrate the existence ofthis beta-phase or of the eutectoid temperature in the nickel-chromiumalloy phase diagram, and there is no suggestion in this specificationthat such a heat-treatment would render the alloys workable or improvetheir high-temperature properties after hot-working to break down thecast structure.

The present invention is based on the discoveries that certain two-phasenickel-chromium base alloys having compositions within acritically-controlled range can be worked to form wrought products afterbreaking down their cast structure, and that the resulting wroughtproducts can be heat-treated to develop surprisingly improvedstress-rupture strengths at 600 C. and above, while retaining excellentductility, by solution-heating within a narrow temperature rangeimmediately below the solidus temperature.

According to the invention the alloys contain, by weight, from 47 to 65chromium, from 0 to 12% cobalt, from 0.02 to 0.1% carbon, from 0 to0.01% boron, one or more of titanium, aluminum, molybdenum, tungsten,tantalum and niobium in amounts in the ranges from 1 to 6% titanium,from 0.5 to 5% aluminum, from 1 to 10% molybdenum, from 2 to tungsten,from 2 to 10% tantalum and from 0.5 to 10% niobium, and such that 3(pe1'cent Ti) +3 (percent Al) +5 X (percent Mo) +2.5 X (percent W) +2.5(percent Ta) +5 X (percent Nb)-1220 with the provisos that (i) the sumof the aluminum and titanium contents does not exceed 6%;

(ii) the sum of the aluminum, titanium, niobium and tantalum contentsdoes not exceed and (iii) the sum or the tungsten and molybdenumcontents does not exceed 10% and one or more of zirconium, cerium,yttrium and hafnium in a total amount exceeding that required to combinewith all the nitrogen present and sufiicient to form a eutectic with thenickel, but not exceeding 4%, the balance, except for impuritiesincluding nitrogen in an amount not exceeding 0.1%, being nickel in anamount of at least 25%. The alloys are hot-worked, preferably byextrusion, to break down their cast structure, and then subjected to asolution heat-treatment at a temperature within the range of 50 C. belowthe alloy solidus temperature for from /2 to hours, if desired afterfurther hotor cold-working to the desired shape. Advantageously they aresubsequently subjected to an aging heat-treatment at a temperature inthe range 600 to 900 C. for from 4 to 20 hours. This latter aging willin any event occur when the alloys are heated above 600 C. in service,but preferably it is performed as a separate step before they are putinto service.

The resulting alloys have an excellent combination of sterss-rupturestrength and ductility at temperatures of 600 C. and above combined withexcellent resistance to the combustion products of impure fuelscontaining sodium, sulphur and vanadium. This makes them particularlysuitable as materials for load-bearing furnace or turbine parts exposedin use to the combustion products of such low-grade fuels.

The chromium content of the alloys must be at least 47% for adequateresistance to the combustion products of low-grade fuel oil containingvanadium, sodium and sulphur as impurities. However, with more than 65%chromium the hot-workability of the alloys becomes inadequate and thereis a tendency for the corrosion resistance to decrease. Increasing thechromium content also lowers the room-temperature ductility, andpreferably therefore the chromium content does not exceed 53%. Theroomtemperature ductility is also adversely affected if the nickelcontent is reduced below Cobalt increases the stress-rupture lives ofthe alloys. However, more than 12% cobalt leads to inadequate corrosionresistance and preferably the co'balt content does not exceed 5%.

At least 0.02% carbon is required so that the alloys possess goodstress-rupture properties but more than 0.1% leads to lowerroom-temperature ductility, tends to reduce corrosion resistance and canlead to difficulties in working, e.g. by forging.

The elements titanium, aluminum, molybdenum, tungsten, tantalum andniobium make an important contribution to the stress-rupture strength ofthe alloys, and one or more of these elements must be present inindividual amounts of at least 1% titanium, 0.5% aluminum, 1%molybdenum, 2% tungsten, 2% tantalum and 0.5% niobium, the total amountbeing such that the relationship 3 x (percent Ti) +3X (percent Al) +5(percent Mo) +2.5 X (percent W) +2.5 X (Percent Ta) +5 X (Percent Nb) 120 is obeyed. However, the presence of individual amounts of theseelements exceeding 6% titanium, 5% aluminum, 10% molybdenum, 10%tungsten, 10% tantalum or 10% niobium embrittles the alloys and reducestheir workability, so that they tend to crack, for example when forged.For the same reasons, the total aluminum and titanium content must notexceed 6%, the total content of aluminum, titanium, niobium and tantalummust not exceed 15% and the sum of tungsten and molybdenum must notexceed 10%.

The presence of uncombined nitrogen in the alloys is highly detrimentalas it leads to embrittlement of the alloys during service. The elementszirconium, cerium, yttrium and hafnium each have a very strong afiinityfor nitrogen, and the alloys must contain one or more of these elementsin an amount in excess of that required to combine with all of thenitrogen present but not more than 4% in all by weight. The excessamounts of these elements form a eutectic with nickel which serves toimpart good workability to the alloys. The preferred eutecticformingelement is zirconium, but cerium, yttrium or hafnium or a combination ofany two or more of these four elements may be used up to the total of4%, although preferably the total amount does not exceed 2%.

The nitrogen content of the alloys must in any event not exceed 0.1%.Nitrogen is commonly introduced into the alloys by the chromium, ascommercial chromium usually contains about 0.1% nitrogen, so thatnitrogen is invariably present in alloys of this type as an impurity. Toachieve a low nitrogen content it is preferable to make the alloys byvacuum-melting, when nitrogen levels of the order of 0.01 to 0.08% canreadily be achieved. Preferably, the nitrogen content does not exceed0.04%.

The amount of eutectic-forming element in excess of the amount combinedwith nitrogen as nitride and in excess of any amount which may bedissolved in the nickel or chromium phases of the alloy may convenientlybe referred to as the effective zirconium (or cerium, yttrium orhafnium). If there is no eutectic-forming.element in solution in thenickel or chromium phases, the effective zirconium is that in excess of6.5 times the nitrogen content, the effective cerium is that inexcessive of 9 times the nitrogen content, the effective yttrium is thatin excess of 6 times the nitrogen content and the effective hafnium isthat in excess of 13 times the nitrogen content. The amounts ofimpurities present other than nitrogen should also be as small aspossible. Thus any silicon, iron and manganese present should not exceed0.5% each.

A particularly satisfactory combination of properties is exhibited byalloys containing from 48 to 53% chromium, not more than 2% cobalt, andone or more of the elements titanium, aluminum, molybdenum, tungsten,tantalum and niobium in the ranges Ti from 2.5 to 4.0%, A1 from 3.0 to4.0%, Mo from 1.0 to 6.0%, W from 2.0 to 8.0%, Ta from 2.0 to 7.5% andNb from 0.5 to 7.5%, subject to the relationships set out above. Thecontents of zirconium, cerium and yttrium preferably do not exceed 1.0%each and the hafnium content preferably does not exceed 1.5%

Most preferably the alloys contain both titanium and aluminum in theranges 2.5 to 4.0% titanium and 3.0 to 4.0% aluminum, the combination of3% titanium and 3.5% aluminum being particularly satisfactory.

Provided the composition of the alloys is within the broad range setforth above, they can be hotor coldworked after their duplex caststructure, which comprises the gamma nickel and alpha chromium phases,has been broken down by extrusion or forging. Thus they can be rolled torod or sheet, swaged, upset, drawn to wire or otherwise shaped to thedesired wrought form.

To develop their high-temperature strength properties the wrought alloysmust be solution-heated at a temperature within 50 C. below the alloysolidus temperature for from 1 to 20 hours. Depending on the precisecomposition, the solidus temperature is generally in the range from 1150to 1348 C. This treatment serves to coarsen the grain structure and thusto reduce the area of the grain boundaries. This effect is surprising ina duplex alloy and is not fully understood, since the two phases wouldbe expected to be in substantial equilibrium s0 that there would be nodriving force available for massive diffusion. It is important to use asolution temperature as close as practicable to the solidus temperatureand in any event not more than 50 C. below it, in order to develop themaximum driving force for grain growth.

After solution-heating the alloys may then be subjected to an agingtreatment at a temperature in the range from 600 to 900 C. for from 4 to20 hours. This is particularly advantageous if the alloys contain atleast one of the elements titanium, aluminum, niobium and tantalum whichform precipitable inter-metallic phases that further contribute to thestress-rupture strength of the alloys, but it also serves to bring thealloy structure into equilibrium by dilfusion of nickel and chromiumfrom the saturated alpha and gamma solid solutions. As alreadyexplained, aging will take place during the initial heating in service,but preferably it is performed as a separate step.

If the highest stress-rupture strength is required for service attemperatures exceeding 650 or 700 C., the

I were vacuum-melted and cast as 3 kg. ingots which were enclosed inmold steel cans and extruded to 16 mm. diameter bar at 1120 C. to breakdown their cast structure.

Test pieces machined from the bars were then subjected to one of thefollowing three heat-treatments, of which numbers 2 and 3 were inaccordance with the invention while number 1 was not, because thesolution heating temperature employed was more than 50 C. below thesolidus temperature of the alloys treated.

(1) 4 hr./1150 C., air-cool+16 hr./700 C., air-cool.

(2) 18 hr./1220 C., air-cool+4 hr./1150 C., air-cool;

+16 hr./700 C., air-cool.

(3) 18 hr./1250 C., air-c0ol+4 hr./1150 C., air-cool;

+16 hr./700 C., air-cool.

The results of stress-rupture tests on the alloys under variousconditions of stress and temperature are set forth in Table II.

TABLE I Composition (weight percent) Alloy Ti Al Nb Mo W Ta Co Zr B C NCr Ni 0.3 0.003 0. 04 0. 015 50 Balance 0 0. 3 0.003 0. O4 0. 013 50 Do.

Norm-The solidus temperature of Alloys 4, 6 and 7 was approximately1,290 C. while that of the other alloys was approximately 1,260 G agingtreatment may follow the solution-heating without any intermediatetreatment. However to obtain the maximum ductility with a somewhat lowerstress-rupture strength the alloy may be given an intermediateheattreatment comprising heating in the range 1100 to 1150 C. for from/2 to 8 hours before it is aged.

As explained above, the alloys are preferably vacuummelted. Howeveralloys free from titanium and aluminum may be melted at atmosphericpressure with the use of an inert gas shield and a dry basic slag coverin order to prevent the ingress of nitrogen.

Some examples will now be given.

Twelve alloys having the compositions shown in Table The results inTable II show the remarkably good combination of stress-rupturestrengths with ductility of the alloys heat-treated according to theinvention, at temperatures between 600 and 1050 C. Comparison of thetest results for Alloy No. 4 at 900 C. shows the 10-fold increase instress-rupture life resulting from the use of heat treatment No. 3according to the invention, while the results for Alloys 6 and 7 at 800and 850 C. at the same stress of 7.7 hbar shows that even at the highertest temperature these alloys, when heat-treated according to theinvention, had stress-rupture lives many times greater than thoseobtained at the lower temperature of 800 C. after solution-heating morethan C. below the solidus temperature.

TABLE II.-STRESSRUPTURE PROPERTIES 54 hbar at 54 hbar at 7.7 hbar at15.4 hbar at 10.8 hbar at 7.7 hbar at 600 C. (1) 600 C. (2) 800 C. (1)800 C. (2) 850 C. (2) 850 C. (3)

Percent Percent Percent Percent Percent Percent Life, elonga- Life,elonga- Life, elonga- Life, elonga- Life, elonga- Life, elonga- Alloyhours tron hours tion hours tion hours tion hours tion hours tlon 4.6hbar at 6.2 hbar at 4.6 hbar at 3.1 hbar at 1.54 hbar at 900 O. (1) 900C. (2) 900 C. (3) 1,000 C. (2) 1,050 O. (1)

Percent Percent Percent Percent Percent Percent Life, elon- Life, elon-Life, clan Li elon- Life, elon- Life, elon- Alloy hours gation hoursgation hours gation hours gation hours gation hours gation 15 52 22 13114 00 N ore-ND =Not determined.

The excellent properties of the alloys according to the invention may becompared with those of two commercially-available wroughtnickel-chromium alloys. The first of these, which nominally contained 1%zirconium, 50% chromium, balance nickel, was solution-treated for 2hours at 1140 C., which was within 30 C. of its solidus temperature of1170 C. The second alloy, which nominally contained 0.7% titanium, 0.6%aluminum, 0.4% zirconium, balance nickel, was not heat-treated. Each ofthese alloys had a stress-rupture life of less than 1 hour under astress of 4.6 hbar at 900 C.

The excellent corrosion-resistance of two typical alloys of theinvention, Nos. 13 and 14, are shown by tests in which specimens ofthese alloys and of the first of the and one or more of zirconium,cerium, yttrium and hafnium in a total amount exceeding that required tocombine with all the nitrogen present and sufiicient to form a eutecticwith the nickel, but not exceeding 4%, the balance, except forimpurities including nitrogen in an amount not exceeding 0.1%, beingnickel in an amount of at least to break down its cast structure andthen solution-heating it at a temperature within the range of 50 C.below its solidus temperature for from /2 to 20 hours.

does not exceed 6%;

commercially-available alloys mentioned above were half- 5 immersed in amolten salt mixture consisting of 80% A Process accoffilng t0 clalm 1111 Whlch the alloy vanadium pentoxide and 20% sodium sulphate at 900 isSubsequently aged m the temperature range from 600 C. for 300 hours. Thecompositions of the alloys and the to for from 4 0 20 loss in weight,measured in milligrams per square centi- 3. A process according t0 claim2 in Whlch the alloy meter, are set forth in Table III below. It will beseen 20 is heated a'ta temperature in the range from 1100 to 1150 thatthe alloys of the invention had a corrosion resistance C. for from /2 to8 hours after the solution heating but comparable with that of the knownalloy. before it is aged.

TABLE III Composition (weight percent) weilght Alloy Ti Al Zr B o N CrNi (mg/6113 A 1.0 50 Balance 480 13 a 1 5 0.3 0.003 0.04 0.015 50 -.-do437 14 4 0.3 0. 003 0. 04 0.015 50 -d0 380 The alloys in accordance withthe invention may be 4. A process according to claim 1 applied to analloy p y r g articles nd parts requiring ig in which the chromiumcontent is from 48 to 53%, the Strength filllcfillty and COIYOSIOI!resistance, for example cobalt content does not exceed 2% and whichcontains sgiio iis fig 22 :3512:261 )?zgg gghi ggrg ilf ci one or moreof titanium, aluminum, molybdenum, tungample turbine blades, rotors andthe like, and particularly i g ig 29 2 rmges T1 from those which areexposed in use to the combustion products 0 mm to 0 mm to of low-gradehydrocarbon fuels, especially those contain- W from to 80%, Ta from toand Nb from ing vanadium, sodium and sulphur. to

Although the present invention has been described in 5. A processaccording to claim 4 in which the contents conjunction with preferredembodiments, it is to be under- 40 of zirconium, cerium and yttrium donot exceed 1% and igozgthtgaltt nocliriifratigrig anllevagilattionsdmtagbe f ii 'i the hafnium content does not exceed 1.5%.

ep g m siianspeo emvention as those skilled in the art will readilyunderg g g i :i j i s; to an alloy stand. Such modifications andvariations are considered 1 amum an a ummum e ranges to be within thescope and purview of the invention and to mamum and to alummumappendedclaims. R f d We claim: e erences He 1. A process of producing a wroughtcorrosion-resistant UN TE STATES PATENTS nickel-chromium alloy producthaving high strength and 2 809 139 1 95 ductility at elevatedtemperatures which comprises hot- 3,519,419 2x 3 g g at 148 13 1 son eta1. 75-171 working a two-phase nickel-chromium alloy containing, 3 627511 12/1971 Ta lot t l 75 by weight, from 47 to 65% chromium, from 0 to12% 1 3 1,1972 e 122 cobalt, from 0.02 to 0.1% carbon, from 0 to 0.01% 075 l76 boron, one or more of titanium, aluminium, molybdenum, 7/1972Mlto et a] 75.176 tungsten tantalum and niobium in amounts in the rangesfrom 1 to 6% titanium, from 0.5 to 5% aluminum, from OTHER REFERENCES 1to 10% molybdenum, from 2 to 10% tungsten, from A y Digest; il y 50Cr/50 Ni, published y s 2 to 10% tantalum and from 0.5 to 10% niobium,and neering Alloys Digest, Inc., Upper Montclair, NJ. such that, 3X(percent Ti) +3X (percent A1) +5X (percent Mo) WAYLAND W. STALLARD,Primary Examiner +2.5X (percent W) +2.5X (percent Ta) +5 (PercentNb) 12o US. Cl. X.R.

75-176; 148-127 with the provisos that (i) the sum of the aluminium andtitanium contents

2. TO 10% TUNGSTEN, FROM 2 TO 10% TANTALUM AND FROM 0.5 TO 10% NIOBIUM,AND SUCH THAT